1. Field of the Invention
The present invention relates to a color conversion definition creating method of creating a color conversion definition for converting coordinate points in a color reproduction area of a device (for example, a printer) that mediates between an image and image data, in a three-dimensional color space (a RGB color space) wherein R (red), G (green), and B (blue) are established as axes, which depends on the device, into coordinate points in a color reproduction area of a print in a four-dimensional color space (a CMYK color space) wherein C (cyan), M (magenta), Y (yellow), and K (black) for printing are established as axes, a color conversion definition creating apparatus, and a color conversion definition creating program storage medium storing a color conversion definition creating program which causes an information processing apparatus such as a computer to operate as the color conversion definition creating apparatus as mentioned above.
2. Description of the Related Art
Hitherto, as an apparatus for applying a good quality of color processing for printing to image data representative of an image, there is known an apparatus that receives CMY data representative of a combination (coordinate points in a CMY color space) of density values of C, M, and Y, and outputs CMY data representative of a combination (coordinate points in a CMYK color space) of dot % of C, M, Y and K (for example, Japanese Patent Reference TokuKai Hei. 9-83824).
This apparatus relates an apparatus for performing processing input CMY data. With respect to such an apparatus, a technology is basically established, while there are proposed various improvements. And there exist many skilled persons who can operate such an apparatus to perform a good quality of color processing (this color processing is referred to as a “setup”).
Recently, as technology of a color management has come into wide use, there is mounting necessity that CMYK data for a good quality of printing is obtained in accordance with color data other than CMY data. By way of an example, it happens that there is a need that upon receipt of RGB data representative of a combination (coordinate points in an RGB color space) of values of R, G, and B, there is printed an image that reproduces colors of a printed image obtained through printing by some printer based on the received RGB data.
When the RGB data is converted into the CMYK data, there is a need not only that the RGB data is converted into the CMYK data capable of obtaining the same colors on a calorimetric basis, but also that the RGB data is converted into the CMYK data excellent in printability. As a parameter of the printability, a K-value is raised. When the RGB data is converted into the CMYK data capable of obtaining the same colors on a calorimetric basis, it is necessary to determine the K-value in accordance with a printing company and a printing machine (K-plate restraint conditions).
Even if various technologies are used so as to convert the RGB data into CMY data that is excellent in printability and is the same colors on a calorimetric basis, a color matching between a color of an image outputted from a specific printer in accordance with the RGB data and a color of an image reproduced in printing is implemented only on an area in which a color reproduction area of the printer is overlapped with a color reproduction area of the printing. And thus, when the color reproduction area of the printer, that is, the outline of the printer profile, is greatly different from the color reproduction area of the printing, that is, the outline of the printing profile, while the outline of the printing profile is usually narrower, it is a problem as to how the color of the color reproduction area of the printer is converted into the color reproduction area of the printing so that an image, which is concerned with a color that is extremely close to the color of the image outputted from the printer in accordance with the RGB data and does not bring about a feeling of wrongness in color tone, is reproduced in printing (this is referred to as a gamut mapping).
With respect to a technology of the gamut mapping, there is proposed an excellent technology (cf. Japanese Patent Reference TokuKai 2001-103329). Japanese Patent Reference TokuKai 2001-103329 discloses a technology in which a direction of the mapping is determined on the device color space (for example, a device-dependence RGB color space) and an actual mapping is performed on the common color space such as an L*a*b* color space. The adoption of this technology makes it possible to cope with both fidelity in colorimetry in vicinity of a gray axis and expression of high coloring in vicinity of a surface of the gamut (a color reproduction area).
However, according to the technology proposed in Japanese Patent Reference TokuKai 2001-103329, the direct use of the technology makes it difficult to perform mapping of the RGB data into CMYK data including a K-value. And thus as disclosed in Japanese Patent Reference TokuKai 2004-007373 for instance, it is considered that an additional device for dealing with RGB data, which has a color reproduction area that is sufficiently coincident with a color reproduction area of printing, is interposed between input RGB data and CMYK data for printing, so that the gamut mapping according to the technology disclosed in Japanese Patent Reference TokuKai 2001-103329 is carried out between the input RGB data and RGB data of the additional device, and thereafter a color matching taking into consideration K-plate restraint conditions is carried out between the RGB data of the interposed device and the CMYK data for printing. Further, according to the technology of Japanese Patent Reference TokuKai 2004-007373, there is a need to actually prepare a device having a color reproduction area that is sufficiently coincident with a color reproduction area of printing. And thus according to a technology of Japanese Patent Reference TokuKai 2004-102489, it is proposed that there is performed an operation virtually preparing a device having a color reproduction area that is sufficiently coincident with a color reproduction area of printing, without actually preparing an additional device, so that the gamut mapping is carried out from the input RGB data to RGB data of the virtual device and a color matching is carried out between the RGB data of the virtual device and CMYK data of printing taking into consideration K-plate restraint conditions.
However, according to the technology of Japanese Patent Reference TokuKai 2004-102489, there is simply prepared for the device having a color reproduction area that is sufficiently coincident with a color reproduction area of printing. But, in the examination of the technology more in detail, the color reproduction area of RGB data, that is, (R,G,B)=(0,0,0)−(255,255,255), is concerned with a regular hexahedron, where the value 255 is the maximum. And when the RGB data is mapped to, for example, the L*a*b* color space, vertexes are eight. To the contrary, in case of CMYK data, there exist black represented by (C,M,Y,K)=(100,100,100,100) where C,M,Y,K represent dot % and the value 100 represents 100% in dot %, that is, the maximum, and in addition, around the black, black of redness, black of greenness, and black of blueness, such as (C,M,Y,K)=(0,100,100,100), (100,0,100,100), (100,100,0,100), respectively. Thus, CMYK data is concerned with many vertexes (generally 14 pieces of vertexes) more than RGB data. Accordingly, in case of a printer that deals with RGB data, it is strictly impossible to implement the same color reproduction area as the color reproduction area of printing. Thus, it is a problem how the difference is harmonized. Further, according to the technology of Japanese Patent Reference TokuKai 2004-102489, there is simply conceptually prepared a virtual device having a color reproduction area that is sufficiently coincident with a color reproduction area of printing, and there is prepared no specific profile of the virtual device. The profile of the virtual device may cause the adaptation to technology of the gamut mapping proposed in the above-mentioned Japanese Patent Reference TokuKai 2001-103329 to greatly vary. Accordingly, it is also an important problem as to how the profile of the virtual device is specifically defined.
Furthermore, with respect to the K-plate restraint conditions, according to the technology of Japanese Patent Reference TokuKai 2004-102489, the K-value is determined from the minimum value of C, M, Y, and thus it is possible to obtain a result somewhat satisfied on the gray axis or the vicinity of the gray axis. However, there is such a problem that it is impossible to represent particularly dark color of chroma saturation. It is not sufficient that the K-plate restraint conditions are simply satisfactorily kept. There is such a problem that even if a grey of color tone, in which four plates of CMYK are overlapped with one another, keeps monotone, it is difficult for an operator to accept that when there occurs the inversion of the tone on any of the plates. For this reason, there are needs to well keep the K-plate restraint conditions on the gray axis and in addition to prevent the four plates of CMYK from bringing about the inversion on the gray axis.
Japanese Patent Reference TokuKai 2005-268982 proposes a technology in which it is intended to solve the above-mentioned problems, and a profile of the virtual device is “suitably” defined, so that a virtual device having a color reproduction area close to the color reproduction area of printing is prepared on an algorithm basis and the K-plate restraint conditions are properly reflected on the gray axis. According to the technology disclosed in Japanese Patent Reference TokuKai 2005-268982, a combination of the technology disclosed in Japanese Patent Reference TokuKai 2001-103329 for instance makes it possible to make a color conversion definition from the RGB color space to the CMYK color space, which is faithful in colorimetry and is free from a tone fault, wherein the K-plate restraint conditions are faithfully reflected on the gray axis.
However, the technology disclosed in Japanese Patent Reference TokuKai 2005-268982 involves a problem that the tolerance to the K-plate restraint conditions is not necessarily enough. More in detail, in the event that there is set up K-plate restraint condition in which the amount of K-plate rises rapidly from a bright color to a dark color, there is a possibility that the value of K of K>0 is allotted to the point that corresponds to the high brightness color that can be reproduced only with K=0 in the color reproduction area as a result of the interpolation, and the high brightness color originally intended can not be reproduced in the print.
In order to solve such an inconvenience that the high brightness color can not be reproduced in the print, Japanese Patent Application Serial No. 2005-131936 proposes technology that the value of K of K>0 is allotted for an area near the white (W) of a surface of the color reproduction area to perform an interpolation operation taking as boundary conditions this allocation.
However, according to the technology proposed in Japanese Patent Application Serial No. 2005-131936, ink total amount limitation (sum total of the dot % of C, M, Y, and K of each point) is not considered. Therefore, it is difficult to apply such technology to the CMYK device that is severe in the ink total amount limitation, such as printers that especially print the newspaper as it is. Moreover, according to the technology proposed in Japanese Patent Application Serial No. 2005-131936, it is also difficult to arbitrarily designate CMYK value of a black spot. Therefore, it doesn't become an enough answer for the user who wants to follow CMYK values of a black spot of printing conditions conventionally used as it is, and wanting remove the color balance of the shadow from a neutral gray intentionally.